The invention concerns a wheel bearing, which enables conclusions to be drawn as to the forces which act between the road surface and the tires of a motor vehicle particularly by measuring the forces currently acting within the stationary ring of the wheel bearing.
To improve the safety and traveling comfort of modern vehicles, the electronic control systems require increasingly more information about the current driving situation for the operation of the vehicle. In addition to the current data on the engine, the gearbox and the rotational speed of the individual wheels, the current forces and force directions (wheel contact forces) which act on the individual tires will also be employed in future for control of vehicle operation. There have been various attempts to record these forces: e.g. measuring forces in the tire or inserting measurement adaptors between the wheel rim and the acceptance flange (DE 196 27 385 or xe2x80x9cErfassung der mehr axialen Fahrbetriebsbelastungen mit dem Mexcex2rad xe2x80x9cVelosxe2x80x9dxe2x80x9d, A. Rupp, W. Diefenbach, V. Grubisic, ATZ Automobiltechnische Zeitung 96 (1994)). These attempts at a solution all have the problem that the forces must be transmitted from a rotating part. For this purpose, additional complexity is necessary, such as the employment of telemetric data transmission or the use of sliding contacts. In this connection, one obvious possibility for recording these wheel contact forces is to use the wheel bearings, because these transmit to the vehicle framework all the forces which act on the tire.
EP 0 432 122 A2 shows a wheel bearing unit in which sensors are arranged at various locations on the stationary ring. The forces which act on the tire can be determined from the forces measured on the stationary ring of the wheel bearing during travel in a straight line or travel round a bend. Despite the large number of measurement locations, the problem with this measurement arrangement appears at the moment when the vehicle brake is actuated. Tests have shown that the wheel contact forces can no longer be exactly determined during the operation of the brake.
The invention has the object of indicating a measurement arrangement for a wheel bearing of a motor vehicle, from which measurement arrangement the wheel contact forces can be determined, even during braking.
This object is achieved by the invention.
The forces which act between the tire and the road surface are measured indirectly by means of the reaction forces in the wheel bearing. For this purpose, sensors (for example strain gauges or thin-film strain gauges) are arranged on the stationary ring of the wheel bearing and the reaction forces (the stress change) in the stationary ring of the wheel bearing are determined by these sensors. The wheel contact forces can be determined directly from these loads during travel without braking. During a braking process, however, the reaction forces in the wheel bearing are so strongly influenced by the forces introduced via the disk brake that the wheel contact forces can no longer be exactly determined. Only knowledge of the position and magnitude of the currently acting forces during the braking process makes it possible to draw conclusions about the forces and the current coefficient of friction between the road surface and the tire from the total forces which are recorded by the sensors on the stationary ring of the wheel bearing.
The spatial position of the center of gravity of the braking forces during the braking process can be continuously determined by the continuous measurement of the forces at the suspension points or in the support members of the flange-connected disk-brake caliper (center of gravity of the braking forcesxe2x80x94calculated point of action of the vector of the braking forces on the brake disk or on the brake linings). The position of the center of gravity of the braking forces depends on the braking force introduced, the current coefficient of friction and the contact relationships between the brake lining and the disk brake. The current center of gravity of the braking forces can therefore be displaced both during a braking process and during the operating period. After reduction of the measured forces in the wheel bearing by the influence of the braking forces, it is possible to use the resulting wheel contact forces to carry out the braking process better than was possible with the prior art.
A further surprising advantage of the measurement arrangement according to the invention relative to EP 0 432 122 A2 and all the measurement wheels, is that for tangentially introduced wheel forces, the braking forces and the influences due to the road surface can be computationally differentiated by means of this measurement arrangement. If a vehicle travels over a soft subsurface (snow, gravel, soft ground, puddle), an accumulation of material forms in front of the tire and this must be pushed away. Pushing away in this manner causes a tangential force in the tire which has the same effect as the braking force. Due to the recording of the current braking forces, these horizontally acting forces can be unambiguously subdivided into braking forces and into forces which act from the road surface onto the tire. This advantage is all the more important because none of the measurement systems, which carry out measurements in the region of the wheel rim and of the tire, can differentiate between these forces. This possibility optimizes the braking process in the soft subsurface.
In a braking process of a driven wheel, furthermore, the forces which act in the brake disk can be differentiated from the driving forces (or driving torque), which are introduced via the drive train. This differentiation is necessary to permit optimum braking of these wheels when the clutch is not released (i.e. there is engine braking). This differentiation of the forces into driving forces from the drive train and braking forces is impossible with all the systems which carry out measurements in the wheel rim or in the tire.
The advantage of the arrangement of the sensors lies in the fact that all the sensors are arranged on a stationary component. This has particular advantages during the manufacture and fastening of the sensors. It avoids the problems of running cables to different components.